AMSTETTEN, Austria  An Austrian man who held his daughter captive for 24 years and fathered her seven children repeatedly warned his captives that they would be gassed if they tried to escape, a spokesman for investigators said Thursday.

The revelation came as authorities also said that Josef Fritzl forced his captive daughter to write a letter last year indicating he may have been planning to release her from the windowless dungeon where she lived with three of their children.

Police Col. Franz Polzer said Elisabeth Fritzl wrote in late 2007 to her family, who believed she had fled to a cult, that she wanted to return home but "it's not possible yet."

DNA testing on the letter proved that 42-year-old Elisabeth had written it, but she was forced to by her father, Polzer said.

"He may have had plans to end the captivity at some point," Polzer told The Associated Press. "It shows that he must have had a spark of humanity."

Fritzl's elaborate crime came to the attention of authorities April 19 when one of Elisabeth's daughters, 19-year-old Kerstin, was admitted to a hospital with an illness linked to an unidentified infection.

Baffled doctors appealed on television for Kerstin's mother to come forward because they needed information about her daughter's medical history. Fritzl then allowed Elisabeth to go to the hospital, and her story came to light.

A hospital spokesman said Thursday that Kerstin was in life-threatening condition. She was brought to the hospital unconscious and later suffered seizures. Now she remains in an induced coma and on a respirator, and is still undergoing dialysis because of the effects of a lack of oxygen, spokesman Klaus Schwertner said.

Meanwhile, Helmut Greiner, a spokesman for federal investigators, told the AP that officers were checking whether Fritzl had indeed set up a mechanism that could send gas into the dingy, windowless cellar as the suspect claimed during questioning.

Experts were also checking another Fritzl claim that the reinforced door leading to the enclosure had a timer that enabled it to be easily opened if he was gone for an unusually long period of time, said Greiner, spokesman for the Federal Bureau of Investigations.

A former tenant of Fritzl's, meanwhile, said he occasionally heard suspicious noises from the area of confinement during his 12 years of residency starting in 1995.

Alfred Dubanovsky said he occasionally heard "knocking, banging" and what sounded like objects being dropped while living in a ground floor apartment of the building owned by Fritzl and above the basement dungeon.

He told the AP that Fritzl told him the noise came from the basement's gas heater.

Days before Elisabeth told police of her 24 years in captivity, law enforcement officials had become suspicious and started investigating Fritzl, according to a police statement.

It said police decided to compare DNA samples of the hospitalized Kerstin and of Fritzl, along with family members living with him, about 10 days before revealing the ordeal to the public. Fritzl and Elisabeth were detained April 26 near the hospital where Kerstin was being treated and Elisabeth then told her story to interrogators.

The letter is one piece of evidence helping authorities piece together Josef Fritzl's double life as both reputable citizen and "horror father" who allegedly held his own daughter and three of their children captive.

One of the children she bore him died as an infant, and Fritzl has confessed to tossing the corpse into a furnace. He managed to smuggle the other three out to be raised by him and his wife, who apparently was led to believe Elisabeth had given them up.

The case has shocked Austria, which is still scandalized by the 2006 case of Natascha Kampusch, who was kidnapped at age 10 and imprisoned in a basement outside Vienna for more than eight years.

Kampusch, who has started a charitable organization to help the Fritzl family, said she believed that Austria's Nazi past might have played some role.

"I think (abuse) exists worldwide, but I think it's also a ramification of the Second World War and its connection to education and so on," Kampusch said in an interview late Wednesday with the British Broadcasting Corp.

She said that during the Nazi era "the suppression of women was propagated. An authoritarian education was very important."

Police are also examining whether Fritzl was also responsible for an unsolved murder in the nearby lakeside village of Mondsee two decades ago, where he and his wife owned an inn and camping ground.

Compiling a complete profile of Fritzl has been difficult because he is refusing to undergo more questioning, police say.

The father faces up to 15 years in prison if convicted on rape charges, the most grave of his alleged offenses. However, prosecutors are investigating whether he can be charged with "murder through failure to act" in connection with the infant's death, a crime punishable by up to 20 years in prison.

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Associated Press writers George Jahn and Melissa Eddy in Vienna, Austria, contributed to this report.

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

A woman sells vegetables at a weekly market in Amstetten, Lower Austria , Wednesday, April 30, 2008 while the crime of incest and abuse is still the talk of the town. A man had held his daughter captive in a dungeon of his house for 24 years, abusing her over decades and fathering seven children with her. (AP Photo/Diether Endlicher)